Doped and undoped silicon dioxides, which are commonly referred to as silicate glasses, are widely used as dielectrics in integrated circuits. Although silicon dioxide possesses a tetrahedral matrix which will impart a crystalline structure to the material under proper heating and cooling conditions, the silicon dioxides used as dielectrics in integrated circuits are typically amorphous materials. Chemical vapor deposition of silicate glasses has become of paramount importance in the manufacture of contemporary integrated circuits. For example, silicate glass doped with both boron and phosphorous is widely used as an inter-level dielectric and as a getter material for mobile sodium ions. As geometries are shrunk in the interest of faster operating speeds and lower manufacturing costs, capacitive coupling between narrowly-spaced conductive layers and structures within the circuits has become a significant performance-limiting problem.
In order to reduce capacitive coupling within integrated circuitry, glasses having lower dielectric constants have been tried. Fluorine-doped glass is particularly interesting because the incorporation of fluorine into the tetrahedral silicon dioxide matrix significantly lowers the dielectric constant of the material. Fluorine-doped glass may be deposited by the plasma reaction of ethyl hexafluoride (C.sub.2 F.sub.6), methyl tetrafluoride (CF.sub.4), or nearly any fluorine-containing compound with tetraethylorthosilicate and diatomic oxygen gas (O.sub.2). Films deposited via this technique have proven to be unusable because they characteristically contain free fluorine atoms. In the presence of humidity, these free fluorine atoms leach out of the glass and combine with water molecules to form hydrofluoric acid. As the acid rapidly attacks most metals, with aluminum being particularly vulnerable to attack, it can cause circuit unreliability and failure.
It is the principal object of the present invention to provide an improved method for forming fluorine-doped silicate glasses having low concentrations of free fluorine atoms.